1. Field of the Invention
The invention relates to an image forming apparatus such as copying apparatus, printer, or the like and, more particularly, to an image forming apparatus for laminating a toner layer onto an intermediate transfer member.
2. Related Background Art
In an electrophotographing apparatus, since a life of a photosensitive member is generally shorter than that of the apparatus main body, when the life of the photosensitive member has expired, the photosensitive member is exchanged to a new one.
It is, therefore, necessary to judge whether the life of the photosensitive member has expired or not.
FIG. 12 shows a mechanism for detecting a life of a photosensitive member from a current flowing to the photosensitive member.
As shown in FIG. 12, a bias voltage 121 is applied to a primary charging member (charging roller) 17. A conductive substrate of a photosensitive member (photosensitive drum) 15 is connected to the ground. A capacitor is equivalently formed by the charging roller 17 and the conductive substrate. A voltage is detected with a detecting resistor 122 from a current Idc flowing to the capacitor. A film thickness d is measured by a detecting circuit 123 on the basis of the detected voltage.
Namely, the current Idc flowing to the surface of the photosensitive drum 15 is expressed by EQU Idc=(.DELTA.C/.DELTA.t).times.Vd. .DELTA.C=.epsilon.S/d
(.epsilon.: dielectric constant, S: contact area of the charging roller and the photosensitive drum, d: film thickness of photosensitive drum). From the above both equations, EQU d=.epsilon.S.multidot.Vd/(Idc.multidot..DELTA.t).
However, in the above life detecting device for detecting the life by measuring the film thickness of the photosensitive drum 15, the current Idc is a very small current, and hence in order to increase a film thickness detecting precision, parts of a very high amplification and a very high precision are needed, thus the apparatus is easily affected by noises and often operates erroneously. Therefore, a high precision and a high noise resistance are required for the life detecting device. There is a problem such that the costs rise in association with it.
As a simple method of life detecting, therefore, it is considered to measure the number of revolutions of the photosensitive drum 15 and to store it as life information in a memory.
However, when a diameter of the photosensitive drum is reduced, the number of revolutions increases and a large memory capacity is needed.